1. Field of the Invention
This invention relates to a reference point potential compensating circuit for use in phase controllers, and especially to a compensating circuit in which the reference point potential of a turn-on element is varied in response to fluctuation in power supply frequency and/or power supply voltage, so that the phase-controlled wave form is not affected by the fluctuations in the power supply.
2. Discussion of the Prior Art
In controlling a load by phase control, for example, in phase control type temperature adjusters, a cosine wave form (See FIG. 1-a) or saw-tooth wave form Vc obtained from an a-c power supply by an integration circuit is superposed on a control voltage Vd representing the temperature of the load. The resulting wave form is compared with a constant voltage Vp and a pulse is produced at a coinciding point A to obtain a load voltage having wave form V.sub.L shown in FIG. 1-b. Many phase control circuits are known for switching a signal to a load to obtain a load voltage signal as in FIG. 1-b. These phase control circuits typically include a thyratron-type element which, in response to the before-mentioned pulse, switches a control signal at some selected phase angle thereof across the load to be regulated. The phase angle determines the amount of voltage applied to the load. Referring to FIG. 1, point A is moved with any amplitude change of control voltage Vd, which, in turn is caused by temperature change. Thus, the load voltage V.sub.L can be controlled. If the circuit is designed for a 60 Hz a-c power supply and if it is desirable to connect it to a 50 Hz a-c, the cosine wave form changes from the solid line to the broken line of FIG. 2-a due to a difference in integration action, and as a result, the phase control wave form varies as shown in FIG. 2-b. That is, since the period T.sub.60 in 60 Hz is smaller than the period T.sub.50 in 50 Hz, the maximum voltage of Vc.sub.60 is smaller than the maximum voltage of Vc.sub.50, establishing a relation. ##EQU1## Thus, 50 Hz as compared to 60 Hz gives a greater effective value of phase control voltage for the control voltage Vd. Hence in temperature adjusters, the device cannot be used with both 50 and 60 Hz power sources.
Also when the power supply voltage fluctuates, a fixed reference point voltage results in a similar inconvenience. When the power supply voltage increases, the point A shifts to A' increasing the turn-on angle, and the load voltage increases markedly with this increase in voltage. In order to maintain the load voltage constant, the turn-on angle should be narrowed.